Electroplating is a well known process for applying metal coatings to an electrically conductive substrate. The process employs a bath filled with a metal salt containing electrolyte, at least one metal anode and a source of direct electrical current such as a rectifier. A workpiece to be plated acts as a cathode. While processes for plating some metals, notably chromium, employ insoluble anodes such as lead alloy, most processes utilize soluble anodes of the metal being plated.
In a typical plating operation, a series of metal anodes are hung from one or more anode bus bars while workpieces to be plated are immersed in the plating bath and attached to a cathode bus bar. The negative terminal of a DC power supply is connected to the cathode bus bar while the positive terminal of the power supply is connected to the anode bus bar. The voltage is adjusted at the power supply to provide a current density on the cathodic workpieces which is considered optimal.
The metal anodes dissolve with use and are replaced from time to time. In many electroplating operations, the dissolved metal concentration in the electroplating solution has a tendency to increase beyond the concentration considered optimal for electroplating, due to the fact that the cathode efficiency is less than the anode efficiency. In other words, metal dissolves from the anodes faster than it plates at the cathodes.
U.S. Pat. No. 4,778,572 (Brown) shows a method of resolving this imbalance between anode and cathode efficiencies. According to this invention, some of the soluble anodes are replaced with insoluble anodes. These insoluble anodes are connected to the same anode bus bar as the soluble anodes and therefore operate at the same electrical potential or voltage. The amount of insoluble anode material employed is such that the current carried by the insoluble anodes is equal to the amount of current that results in the production of hydrogen gas at the cathode.
One problem with using insoluble anodes is that the electrode potential required for evolution of oxygen at insoluble anodes is greater than the electrode potential required for dissolution of metal from soluble anodes. As a result, the current density obtained from the insoluble anodes is significantly lower than that obtained from the soluble anodes when operated at the same overall applied voltage. The reduction in voltage drop across the solution at the reduced current density compensates for the higher electrode potential. Consequently, a greater quantity of insoluble anode material is required to carry a given current than would otherwise be the case. This problem is exacerbated by the use of ion exchange membranes in conjunction with the insoluble anodes, as outlined in the '572 patent, due to the voltage drop across the membrane. A further disadvantage of the lower current density obtained by insoluble anodes is uneven current distribution and resulting uneven thickness of metal deposited on the cathodic workpieces. The cathodic current density and deposit thickness are somewhat less at locations across from insoluble anodes than would be the case at the same locations if soluble anodes had been employed.
In actual operation the electrode potential is approximately equal to that component of the total potential difference between the anode and cathode which pertains to the reaction at the electrode only. For example, in the case of an anode, the anode electrode potential would exclude voltage losses due to solution resistance and plating of metal at the cathode.
For some electroplating processes, an insoluble anode installed along with soluble metal anodes at the same electrical potential will carry no current whatsoever. This is said to occur with copper electroplating, for example. In such cases, it is necessary to apply a higher electrical potential to the insoluble anodes than to the soluble anodes, in order to obtain current flow through the insoluble anode. This is normally accomplished through use of a second auxiliary power supply in addition to the first primary power supply. One method of employing an auxiliary power supply for this purpose is outlined in Japanese patent application SH056-112500.
An analogous problem occurs with alloy plating systems. Where it is desired to simultaneously electroplate two different metals simultaneously such as iron and nickel to produce an alloy coating, it is necessary to provide a means of replenishing the metal content of the solution. The simplest approach is to hang separate soluble anodes of the two metals on the same bus bar. Such systems are seldom practical unless the metals have approximately the same electrode potential. For most metal combinations, the electrode potentials are different so that one metal is almost sure to act as an inert anode. Different bus bars and rectifiers can be used for each metal. However, it is difficult to maintain the correct amount of current to each anode type as the total current requirements of the bath change with variation in the size of the cathode work load.
In hoist type plating operations, a certain period occurs during the plating cycle, as well as during plant shutdowns, where there are no parts being plated in the plating bath. During this time a battery effect is experienced, whereby a potential is set up between the soluble anodes and the insoluble anodes. The soluble anodes remain anodic while the insoluble anodes take on a cathodic charge. This is disadvantageous, since many insoluble anode electrode substrates such as lead alloys and titanium, depend on the formation and maintenance of a stable oxide film on the electrode surface for corrosion resistance. When charged cathodically, this oxide film breaks down, resulting in corrosion of the anode and premature loss of effective life. For example, failure of iridium oxide coated titanium anodes has been experienced in nickel plating field tests in periods of less than three months when continuous accelerated laboratory life tests had predicted several years life. This premature insoluble anode failure severely restricts or precludes the use of these insoluble anode materials in many cases.